The present invention relates to the formation of dielectric layers. More particularly, the present invention relates to a method for forming a low dielectric constant film that is particularly useful as a premetal or intermetal dielectric layer in an integrated circuit.
Semiconductor device geometries have dramatically decreased in size since integrated circuits were first introduced several decades ago, and all indications are that this trend will continue. Although today's wafer fabrication plants are routinely producing ever-shrinking devices, the plants of the future will soon be producing devices having even smaller geometries.
In order to continue to reduce the size of devices on integrated circuits, it has become necessary to use insulators having a low dielectric constant. Such films are particularly desirable for premetal dielectric (PMD) layers and intermetal dielectric (IMD) layers to reduce the RC time delay of the interconnect metallization, to prevent crosstalk between the different levels of metallization, and to reduce device power consumption. To this end, several semiconductor manufacturers, materials suppliers and research organizations have focused on identifying low and extremely low dielectric constant films. As used herein, low dielectric constant materials are those films having a dielectric constant between 2.5 to 3.0 and extremely low dielectric constant (“ELk”) films are those films having a dielectric constant below 2.5 extending to dielectric constants below 2.0.
One approach for reducing the dielectric constant includes introducing high porosity into the dielectric film layer. The dielectric constant of air is nominally 1. Thus, dielectric films when made porous, tend to have much lower dielectric constants relative to solid dielectric films, and values of dielectric constants less than 2.5 are becoming achievable.
One method of forming a particular type of ELk material is based on a sol-gel process, in which a high porosity film is produced by hydrolysis and polycondensation of a silicon alkoxide such as tetraethylorthosilicate (TEOS). The sol-gel process is a versatile solution process for making both ceramic and organosilicate materials. In general, the sol-gel process involves the transition of a system from a homogeneous liquid “sol” (mostly colloidal) into a solid “gel” phase. The starting materials used in the preparation of the “sol” are usually inorganic salts or compounds such as silicon alkoxides. The precursor solutions are typically deposited on a substrate by spin on methods. In a typical sol-gel process, the precursor is subjected to a series of hydrolysis and polymerization reactions to form a colloidal suspension, or a “sol.” A typical sol-gel process is described in U.S. Pat. No. 6,264,741 to Brinker et al. Further processing of the “sol” enables one to make ceramic materials in different forms. The further processing may include the thermal decomposition of a thermally labile component, which may include the formation of an ordered surfactant-templated mesostructured film by evaporation-induced self-assembly, followed by the thermal decomposition of the template. Another sol-gel process is described in U.S. Pat. No. 5,645,891 to Liu et al.
In a particular sol-gel-based process for forming a porous low dielectric constant film, surfactants act as the template for the film's porosity. The porous film is generally formed by depositing a sol-gel precursor on a substrate, and then selectively evaporating solvent components of the sol-gel precursor to form supramolecular assemblies. The assemblies are then formed into porous films by the pyrolysis of the surfactants at a temperature range between approximately 300 and 450° C. This particular sol-gel-based process can produce porous films with controllable pore size and advantageously, with narrow distributions of pore size, which is beneficial for integrated circuit manufacture.
FIG. 1 is a flowchart illustrating a basic sol-gel-based process that has been previously proposed to deposit ELk films. As shown in FIG. 1, the first step is the synthesis of the stock precursor solution (step 100). The stock precursor solution is prepared, for example, by combining a soluble silicon oxide source (which may be a mixture of silicon oxide precursors) (e.g., tetraorthosilicate (TEOS), methyltriethoxysilane (MTES), etc.), water, a solvent, and an acid catalyst (e.g., nitric or an organic acid) in particular mole ratios at certain prescribed environmental conditions and by mixing the combination for certain time periods.
Once the stock solution is obtained, the coating solution is mixed (step 110). The general procedure for preparing the coating solution includes adding a porogen, such as a surfactant, to the stock solution. For example, porogens such as surfactants can be used as templates for the porous silicon oxide. Later in the process, the porogens are removed (i.e., calcined), leaving behind a porous silicon oxide film. The surfactants can be anionic, cationic, or nonionic. However, when forming dielectric layers for IC applications, non-ionic surfactants are generally preferred. The acid catalyst is added to accelerate the hydrolysis reactions of the silicate and aid in the condensation of the silicate species around the porogen.
After the coating solution is mixed, it is deposited on the substrate (step 120) using a spinning process. Centrifugal draining during the spinning process ensures that the substrate is uniformly coated with the coating solution. The coated substrate is then heated to continue the hydrolysis and condensation of the precursor and to remove low boiling solvents from the forming film (step 130).
The pre-baked substrate can then be further baked to form a hard-baked film (step 140). The temperature range chosen for the bake step will ensure that excess water and higher boiling solvents are removed. At this stage, the film is comprised of a hard-baked matrix of silicon oxide and porogen. An interconnected structure in the hard-baked matrix provides continuous pathways for the subsequently decomposed porogen molecules to escape from the forming porous oxide matrix.
Typical silicon oxide films often have hydrophilic pore walls and aggressively absorb moisture from the surrounding environment. If water, which has a dielectric constant (k) of about 78, is absorbed into the porous film, then the low k dielectric properties of the film can be detrimentally affected. Often these hydrophilic films are annealed at elevated temperatures to remove moisture and to decompose the porogen out of the silicon oxide-porogen matrix. A porous film exhibiting interconnected pores is produced after annealing (step 150). This is only a temporary solution in a deposition process, since the films may still be sensitive to moisture absorption following this procedure.
Some sol-gel processes include further post-deposition treatment steps that modify the surface characteristic of the pores to impart various desired properties, such as hydrophobicity, and increased resistance to certain chemicals. A typical treatment that renders the film more stable is treatment with HMDS (hexamethyldisilizane, [(CH3)3—Si—NH—Si—(CH3)3]) in a dehydroxylating process. HMDS removes hydroxyl groups and replaces them with trimethylsilyl groups to render the film hydrophobic (step 160). See U.S. Pat. No. 5,736,425. Alternatively, or in conjunction with the silylation step, the porous material may be rendered more hydrophobic by the addition of an alkyl substituted silicon precursor, such as methyl triethoxysilane, CH3Si(OCH2CH3)3, (MTES) to the precursor formulation. See EP 1,123,753. It has been found that replacement of a significant fraction of the TEOS with MTES (for example 20-80 wt %, preferably 30-70 wt %) in the liquid precursor formulation generates films exhibiting good resistance to moisture absorption without subsequent exposure to HMDS.
A variety of alternatives to the above-described sol-gel process for depositing ELk materials have been proposed. However, none of the alternatives is suitable for use in integrated circuit fabrication due to the unacceptable impurity types or poorly controlled or uncontrolled impurity levels associated with commercially available surfactant components in the precursor solutions used for porous ELk deposition.
In order for the film to be suitable and allow for a successful integration for IC fabrication, the film desirably has a controlled level of impurities or it is deposited using ingredients that have minimal levels of impurities that are harmful in silicon oxide-based insulator films in microelectronic devices. Impurities that are harmful in silicon oxide-based dielectric films include metal ions such as sodium and potassium which transport under the influence of an electric field and which are nonvolatile. These impurities are typically introduced into the film by the surfactants, which are used in templated porous oxide precursor formulations.
It is known in the semiconductor integrated circuit industry that alkali metal ions (such as sodium and potassium) are desirably excluded from silicon dioxide films used as MOS transistor insulators and multilevel interconnection insulators, because these positively-charged ions are mobile when exposed to electric fields. They drift away from the positively-biased film interface and toward the negatively-biased film interface, causing capacitance-voltage shifts. While the exclusion of sodium has received the most attention because of its ubiquitous presence and high mobility, other alkali ions, especially potassium and lithium, are also equally problematic and are also desirably excluded from insulator films. Alkali metal impurity specifications for chemical precursor solutions (e.g., TEOS) for integrated circuit applications typically set the allowable impurity levels to approximately 20 parts per billion maximum for each type of alkali metal, or other metal. Lower maximum allowable impurity levels may be possible in the future.
Therefore, there is a need to arrive at a formulation, which uses surfactants that do not contain impurities (or contains reduced amounts of impurities), and which can be used to produce good quality low dielectric constant films.